Aaron,
Hopefully you have read the thread by Mark. I can tell you this, dreaming of hunting DG with a double rifle will not go away, nor will it be satisfied by a bolt gun, dreams do not recognize reality, logic, or facts.
The question you have to ask yourself is; "What, exactly will it take to satisfy that dream?"
Will you be satisfied with one-and-done, or do you see a future filled with hunting with a double?
There's a difference between doing something just to say you did or check a box, and really doing it well.
You might see yourself wearing an Armani suit to your high school prom, but borrowing your cousin's, who is 2 inches taller and outweighs you by 40 pounds, doesn't exactly paint the picture you imagined.
To be effective and proficient, a double rifle has to fit you and you need to practice. A PH might let you shoot his double at a target, but I don't see him handing you his rifle to shoot an elephant.
With respect to shooting, there is nothing worse or more disappointing than shooting a rifle or shotgun which doesn't fit you. But, missing a pheasant does not compare to a poor shot on an elephant or cape buffalo.
I shot alongside another hunter whose double was too short and he was high on almost every shot. He was dead on with his bolt gun.
You have gotten some excellent advice from some extremely experienced people. I think it would be safe to say they all understand your dream.
You have to decide where the dream of what you want to do merges with the reality of what you can do.
I do not think I'm alone when I say, once shooting a double gets into your blood - it does not go away. You start looking for ways to use your double. On my recent hunt in Mozambique I spent 3 hours getting into position just so I could shoot a croc at close range with my double.
Be forewarned, they are addictive - I have a Heym 89B 450/400, I pick up my 89B 500NE tomorrow.